The present invention relates to a reverse development method for providing reverse toner images by causing one-component magnetic toners to be attracted to the uncharged areas of a latent electrostatic image formed on an image-bearing member surface, and more particularly to a reverse development method capable of providing an improved image quality by improving the fluidity conveyability of magnetic toners.
Encouraged by wide use of computers in recent years, developments have been made actively on printer as peripheral equipment for producing hard copies as the outputs of information in the form of letters and figures. The printers which were most commonly used conventionally are so-called impact-type printers which produce prints by physically impinging selected printing types upon papers via ink ribbons. However, the appearance of higher-performance computers and the diversification of information to be processed have necessitated the quick processing of large amounts of information and various types of output forms such as various sizes of letters such as Chinese characters and figures. These requirements cannot be met by the conventional impact-type printers, so efforts have been made to develop new nonimpact-type printers (electronic printers).
The nonimpact-type printers ar classified into three groups; an electrophotographic type, an electrostatic type and an ink jet type rom the viewpoint of recording methods. In order to cope with the recent trends of increase in recording speeds and densities, the electrophotographic type appears to be the most promising.
The principles of recording by electrophotographic printers are essentially the same as those of usual copiers: The recording process comprises the steps of uniformly charging a photosensitive member surface, forming a latent electrostatic image by exposure, developing the latent electrostatic image with a toner, transferring the toner image onto a plain paper and fixing. Since in the electrophotographic printer, information supplied from a computer is written on a uniformly charged photosensitive surface with a laser beam, etc., and toner is caused to attach to the written areas, namely the exposed areas of the surface, development should be done in reverse.
Dry developers for the reverse development are usually two component-type developers which consist of magnetic carriers and non-magnetic toners like those for copiers. Most printers now in use utilize such developers.
When the two component-type developers are used, the toners have enough electrostatic charges due to the tribo-electrification with the carriers so that they can produce exact development of the nonimage areas of the latent electrostatic image. Further, since the toners retain electrostatic charges after the development, the toner image can be electrostatically transferred onto a commonly available plain paper, resulting in high-quality print image. The use of the two component-type developers, however, requires means for keeping carrier-toner mixtures at constant mixing ratios to maintain the constant optical densities of the resulting images, resulting in larger and more complicated developing apparatuses. In addition, the mixing and stirring of the carriers with the toners for extended periods of time leads to the formation of toner layers on the carrier surfaces, deteriorating the triboelectric characteristics of the carriers, which requires the periodic replacement of the carriers.
To solve these problems, one component-type developers consisting only of magnetic toner particles as dry developing components for developing latent electrostatic images have been developed and put into practical use. In a reverse development method using the so-called magnetic toners, the toners are generally attracted to the nonimage areas by applying DC bias voltage of the same polarity as that of the latent electrostatic image to a conductive sleeve holding the magnetic toners charged with the same polarity as that of the electrostatic image. Also, to facilitate electrostatic transfer of the developed toner image to a copy sheet, it is general to use insulating magnetic toners having high electric resistivity.
On the other hand, to achieve good development not only of electrostatic images consisting of line images but also of those including picture images, a development roll provided with a large number of microelectrodes (float electrodes) electrically insulated from each other was proposed and put to practical use (Japanese Patent Laid-Open No. 57-114163).
A typical example of a development apparatus comprising this development roll is shown in FIG. 1. 1 denotes a drum for bearing an electrostatic image on its surface and rotating in the direction shown in by the arrow. 2 denotes a development apparatus comprising a toner container 3 and a development roll 4. The development roll 4 is arranged in opposite to the image-bearing drum 1. The development roll 4 is constituted by a conductive core 5, a permanent magnet 6 having 8 magnetic poles and mounted on the surface of the conductive core 5, and a dielectric layer 7 provided around and fixed to the magnet 6. The dielectric layer 7 is provided with microelectrodes or float electrodes (not shown) composed of conductive particles mutually insulated from each other. 8 denotes a blade in contact with the surface of the development roll 4 in its tangential direction. 9 denotes a bias voltage source, one terminal of which is connected with the conductive core 5 and the other with the ground. 10 denotes a magnetic toner contained in the toner container 3, which is attracted to the surface of the development roll 4 and conveyed to a development region near the image-bearing drum 1 by the rotation of the roll 4 in the direction shown by the arrow.
In the above reverse development system, a thin layer of the magnetic toner 10 is formed by the blade 8 in contact with the development roll 4. The blade 8 is generally made of rubbers or other elastic materials. Therefore, smooth contact between the development roll 4 and the blade 8 is not always achieved, causing troubles in conveying the magnetic toner 10. In addition, since the fluidity of the magnetic toner 10 decreases at high humidity, the conveyance of the magnetic toner 10 cannot be conducted smoothly, failing to supply a sufficient amount of the magnetic toner to the development region. Further, particularly when a large number of copies are produced, the image quality is sometimes extremely deteriorated.